The need to reduce the cost and size of electronic equipment has led to a continuing need for smaller filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Many such devices utilize filters that must be tuned to precise frequencies. Hence, there has been a continuing effort to provide inexpensive, compact filter units.
One class of filter element that is capable meeting these needs is constructed from acoustic resonators. These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is suspended in air by supporting it around the perimeter. When an electric field is created between the two electrodes via an impressed voltage, the PZ material converts some of the electrical energy into mechanical energy in the form of sound waves. The sound waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At the mechanical resonance, the device appears to be an electronic resonator; hence, the device can act as a filter. The mechanical resonant frequency is that for which the half wavelength of the sound waves propagating in the device is equal to the total thickness of the device for a given phase velocity of sound in the material. Because the velocity of sound is many orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the gigahertz (GHz) range may be constructed with physical dimensions less than 100 microns in diameter and few microns in thickness.
At the heart of Thin Film Bulk Acoustic Resonators (FBARs) and Stacked Thin Film Bulk Wave Acoustic Resonators and Filters (SBARs) is a thin sputtered piezoelectric film having a thickness on the order of one to two microns. Electrodes on top and bottom sandwich the piezoelectric acting as electrical leads to provide an electric field through the piezoelectric. The piezoelectric, in turn, converts a fraction of the electric field into a mechanical field. A time varying “stress/strain” field will form in response to a time-varying applied electric field.
To act as a resonator, the sandwiched piezoelectric film is suspended in air to provide the air/crystal interface that traps the sound waves within the film. The device is normally fabricated on the surface of a substrate by depositing a bottom electrode, the PZ layer, and then the top electrode. Hence, an air/crystal interface is already present on the topside of the device. A second air/crystal interface is provided on the bottom side of the device.
FIG. 1 illustrates a cross-sectional view of a typical bulk acoustic resonator device 2 having a substrate 3, a bottom electrode 4 disposed on the top surface of the substrate 3, a layer of PZ material 5 covering the bottom electrode 4 and portions of the top surface of the substrate 3, a top electrode 6 disposed on top of the PZ material layer 5, and a bottom electrode electrical contact 7 disposed in an opening of the PZ material layer 5 and in contact with the bottom electrode 4. The box 8 represents the aforementioned second air/crystal interface provided on the bottom side of the device 2, also referred to sometimes as the “swimming pool.” The active region of the PZ material layer 5 is the portion of the layer 5 above the swimming pool 8.
Typically, the bottom electrode 4 is exposed during the process of etching the opening in the PZ material layer 5 in which the bottom electrode electrical contact 7 will be formed. Because the metal of the bottom electrode 4 will not have an infinite selectivity to the etchant, a portion of the metal of the bottom electrode 4 will be consumed during the etch process. For the typical bulk acoustic resonator design shown in FIG. 1, there is typically sufficient bottom electrode thickness after the PZ material layer 5 has etched to form the opening for the bottom electrode electrical contact 7. This is not always the case, especially for very high frequency bulk acoustic resonator devices that us very thin film thicknesses.
FIG. 2 illustrates a cross-sectional view of a bulk acoustic resonator device 12 having a substrate 13, a bottom electrode 14 disposed on the top surface of the substrate 13, a layer of PZ material 15 covering the bottom electrode 14 and portions of the top surface of the substrate 13, a top electrode 16 disposed on top of the PZ material layer 15, and a bottom electrode electrical contact 17 disposed in an opening of the PZ material layer 15 and in contact with the bottom electrode 14. The box 18 represents the aforementioned second air/crystal interface provided on the bottom side of the device 12.
The device 12 shown in FIG. 2 is identical to the device 2 shown in FIG. 1 except that the film stack is thinner such that there is insufficient bottom electrode 14 remaining after the process of etching the opening in the PZ material layer 15 in which the bottom electrode electrical contact 17 will be formed. The insufficient amount of bottom electrode 14 remaining after the etching process can cause a variety of problems, including, for example, poor electrical contact, high series resistance and reliability issues. The etch rate is a fixed selectivity between the metal of the bottom electrode 14 and the PZ material of layer 15. Therefore, when the aspect ratio between the thickness of the PZ material layer 15 and the thickness of the bottom electrode 14 is large, an insufficient amount of bottom electrode 14 can remain after the etching process. In fact, for a very large aspect ratio, it is possible for the etch process to etch completely through the bottom electrode 14.
Accordingly, a need exists for a process to manufacture bulk acoustic resonator devices that ensures that there is sufficient bottom electrode material remaining after performing the etching process to form the opening in the PZ material layer for the bottom electrode electrical contact.